Bisphenol A, 2,2'-bis(4-hydroxyphenyl)propane*, is widely used in the manufacture of epoxy resins and polycarbonates. Bisphenol A is usually prepared by the condensation of phenol and acetone in the presence of an acid catalyst with a sulfur-containing promoter. However, the reaction between phenol and acetone to form 2,2'-bis(4-hydroxyphenyl)propane using any known catalyst always produces a number of by-products. It is well known that the purity of bisphenol A is very important with regard to the quality of polymers which are prepared therefrom. In the production of polycarbonates, bisphenol A purity requirement is much higher than that of the product obtained by any production method without further purification. Typical commercial bisphenol was found to contain 4% o,p'-isomer, 3% trisphenol I and 1% Dianins compound (Anal. Chem. 31, 1214-17, 1959). There are many patents related to the purification of bisphenol; and the extent of the purification necessary is dependent on yield, crude bisphenol purity, and quality of final product desired. One method suggested is the formation of a 1:1 crystalline complex with phenol (U.S. Pat. No. 2,791,616). The phenol complex may be refined by washing with phenol, after which it is remelted and heated under vacuum to decompose the complex and distill out the phenol. FNT *Also called 4,4'-isopropylidene diphenol
A number of suggested processes describe merely leaching crude bisphenol with a solvent or mixture of solvents selected to dissolve maximum amounts of by-products and minimum amounts of bisphenol. However, the bisphenol obtained from such solvent leaching normally is not pure enough for polycarbonate production. The bisphenol can also be purified by a combination of vacuum distillation and solvent leaching techniques (U.S. Pat. Nos. 3,219,549 and 3,290,391). More complicated, but more effective methods involve crystallization from an organic solvent at a temperature and pressure above the atmospheric boiling point of the solvent (U.S. Pat. No. 3,673,262). In yet another process for purification of bisphenols, a mixture of the reaction product, water and a water immiscible organic solvent is heated to a temperature below the boiling point of the organic solvent to provide two liquid phases which are then cooled to crystallize the bisphenol (U.S. Pat. No. 3,535,389). U.S. Pat. No. 3,326,986 employs a similar process in which the crude bisphenol is heated and melted in water without any organic solvent. The melt is agitated, then cooled and the crystals washed with a chlorinated organic solvent, e.g., methylene chloride, to remove the impurities.
Recovery of the purified product by the crystallization procedure varies from about 50 percent to a rarely achieved 90 percent which adds considerably to the cost of the bisphenol product finally obtained. It is well known that the final product purity is inversely related to the yield. Higher product purity will give lower yield. Most literature on bisphenol purification through crystallization emphasizes the final product purity and neglects the product yield. Known processes provide a yield of from 50-95% of the desired p,p'-bis product. The most effective method to increase the yield is to convert the by-products in the residue back to useful bisphenol through acid-catalyzed rearrangement or isomerization in a phenol medium. In order to make such isomerization feasible, the by-product concentration in such residual streams has to be very high. It is an object of this invention to provide a bisphenol purification process with a high yield and a high purity final product. The ultimate object of this invention is to provide an efficient process for the production of high purity 2,2'-bis(4-hydroxyphenyl)propane and achieve a yield high enough to generate the residual stream having a by-product concentration sufficient to make the rearrangement reaction practical.
It has now been discovered that a pure 2,2'-bis(4-hydroxyphenyl)propane, having less than about 0.25% and even as low as 0.02% of the o,p'-isomer, can be prepared by (1) crystallizing the crude bisphenol A in the presence of water and an organic solvent, e.g., toluene, (2) mixing the remaining mother liquor (after stripping off water and toluene) with phenol and thereafter contacting with a cation exchange resin in acid form or hydrochloric acid to rearrange the by-products to the desired product, (3) stripping the phenol and recycling to the crystallizer of step 1 or to the primary bisphenol reactor. If a second crystallization is included following step 1, the lowest level of by-product in the final product can be achieved.